The use of linear modulation is becoming increasingly important for wireless communication systems that are required to operate at a high data transmission speed. A wireless transmitter for use with linear modulation can employ envelope tracking to enable a power amplifier to attain high power efficiency. Envelope tracking sets a special challenge for a power supply that provides an operating voltage for such a power amplifier, because the operating voltage is not constant, but instead varies according to the envelope of a modulated signal being amplified. In order to attain high efficiency of such a wireless transmitter, the power supply must have high power efficiency. The power supply must provide a spurious-free voltage for the power amplifier. Additionally, the power supply must be able to follow a wideband control signal, corresponding to the envelope of the modulated signal being amplified, with high accuracy. This means that linearity of the power supply must be very good.
Conventional switched mode power supplies are operated at a relatively low switching frequency in order to achieve good power efficiency. However, this is not generally possible for a power supply for use with envelope tracking as the required modulation bandwidth is high. For example, the Third Generation Partnership Project (3GPP) Long Term Evolution standard using a 10 MHz bandwidth (LTE10) requires more than 10 MHz bandwidth for the wideband control signal. In general, the switching frequency of a switched mode power supply needs to be at least five times the bandwidth of the wideband control signal.
A typical switched mode power supply is based on the use of an inductor that is switched alternately to a supply voltage, such as a battery voltage, and to ground. The duty cycle of the switching determines the output voltage. The switching employs two switches that must not both be open at same time. If these two switches were to be open at same time, there would be a short circuit between the supply voltage and ground, which may damage the switched mode power supply, and in particular the two switches. To ensure that the two switches are not both open at the same time, a gap, also referred to as a dead time, is provided between switching the inductor from the supply voltage to ground, and between switching the inductor from ground to the supply voltage. However, for high power efficiency, the dead time should be short. There is a trade-off in choosing the dead time. A short dead time is difficult to implement due to integrated circuit temperature and process variations that alter delays in circuitry, and conventionally the dead time should be long enough to encompass such variations. Therefore, there is a requirement for an improved switched mode power supply that can operate at a high switching frequency.